Centrifugal mixing apparatus and method

ABSTRACT

An improved apparatus and method for mixing a second material into a first fluid material includes a container for the first fluid material, a motor driven centrifugal impeller, and a conduit having an outlet coaxial with and adjacent to the centrifugal impeller for feeding the second material into the center of the impeller. The impeller is located in the container of fluid material, and the improved apparatus includes means for directing a portion of the fluid from the container inward for mixing with the second material as it enters the center of the impeller and means for directing at least a portion of the mixture flowing from the impeller outward for dispersion in the fluid in the container. The apparatus preferably also includes means for recirculating a portion of the mixture from the impeller directly back for mixing with and imparting momentum to the first and second materials as they enter into the center of the impeller. An alternate embodiment of the apparatus includes a venturi section coaxially surrounding the conduit for feeding the second material into the center of the impeller and means for recirculating at least a portion of the mixture flowing from the impeller back to the inlet of the venturi section to augment the pressure at the impeller inlet.

United States Patent 91 Hege June 28, 1974 CENTRIFUGAL MIXING APPARATUSAND METHOD [75] Inventor: Douglas Hege, Woodland Hills,

Calif.

[73] Assignee: Hege Avanced Systems Corporation,

Woodland Hills, Calif.

22 Filed: JuneS, 1972 [2l] Appl. No.: 259,678

[52] US. Cl 259/24, 259/8, 26l/87 [5l] Int. Cl. BOlf 5/16, BOlf 7/20,BOlf 15/02 [58] Field of Search 259/8, 23, 24, 43, 44,

259/66, 67, 96, I07, I08; 26l/84, 87

[56] References Cited UNlTED STATES PATENTS 2,875,897 3/1959 Booth26l/87 X 3,233,876 2/l966 Faure et al 259/96 3,393,802 7/1968 Logue etal 261/84 X 3,606,270 9/1971 Zimmerly 259/96 X FOREIGN PATENTS ORAPPLlCATlONS 419,8l2 10/1925 Germany 259/96 Primary ExaminerHarvey C.Hornsby Assistant Examiner-Alan Cantor j Attorney, Agent, or Firm-Kenyon& Kenyon Reilly Carr & Chapin 571 ABSTRACT An improved apparatus andmethod for mixing a second material into a first fluid material includesa container for the first fluid material, a motor driven centrifugalimpeller, and a conduit having an outlet coaxial with and adjacent tothe centrifugal impeller for feeding the second material into the centerof the impeller'. The impeller is located in the container of fluidmaterial, and the improved apparatus includes means for directing aportion of the fluid from the container inward for mixing with thesecond material as it enters the center of the impeller and means fordirecting at least a portion of the mixture flowing from the impelleroutward for dispersion in the fluid in the container.

- The apparatus preferably also includes means for re circulating aportion of the mixture from the impeller directly back for mixing withand imparting momentum to the first and second materials as they enterinto the center of the impeller. An alternate embodiment of theapparatus includes a venturi section coaxially surrounding the conduitfor feeding the second material into the center of the impeller andmeans for recirculating at least a portion of the mixture flowing fromthe impeller back to the inlet of the venturi section to augment thepressure at the impeller inlet.

12 Claims, 11 Drawing Figures PATENTEDJUN28 m4 SL820 1759 SHEET 3 UF 5 IPAIENTEDJUN28 I974 33320 759 sum 5 w 5 CENTRIFUGAL MIXING APPARATUS ANDMETHOD BACKGROUND OF THE INVENTION This invention relates to a methodand apparatus for mixing a second material into a first fluid materialby use of a centrifugal impeller. The second material is usually a dryparticulate material but can also be a liquid or a gas, and the firstmaterial is normally a liquid.

Centrifugal pump mixers or blenders are well known in which dryingredients, liquids, or gases are blended into a liquid by feeding theingredients through a conduit into the inlet of a centrifugal pumphaving an open-face impeller. The liquid is supplied under pressuretangentially into an annular passageway surrounding the conduit and thendirected outward for flow through at least a portion ofthe impellerpassageways before discharge from the pump casing. Some mixing of theliquid and the added ingredients occurs in the pump impeller, but suchmixing is limited because the liquid does not flow inward to join withthe added ingredient in the center of the impeller but instead isdiverted by a flange at the end of the conduit to flow outward betweenthe pump casing and the open-face centrifugal impeller. The pump casingsin these prior art mixers are of conventional volute form and have asingle outlet tangent to the outer circumference. In a typical processapplication, the liquid carrying the entrained added ingredients will beconductedthrough an external pipeline to a separate storage tank.Normally, more thorough mixing is required than results from a singlepass through the mixing pump; so a line will be connected between thebottom of the storage tank and the inlet to the annular passageway forrecirculating a portion of the mixture in the tank back through themixing pump.

Except for mixtures of very low viscosity, such a system will require asecond centrifugal pump in the recirculating supply line for mixtures ofmoderate viscosity (up to approximately percent solids). For highviscosity mixtures, a third support or booster pump may be needed in thedischarge line between the mixer and the storage tank and both thesupply and the discharge recirculating booster pumps may have to be ofpositive displacement type for the thicker mixtures (40-60 percentsolids). The recirculation support pumps, pipe and fittings required foroperation of these prior art mixers not only add significantly to thecost of a complete mixing installation but also create a cleanoutproblem, particularly if recirculation is stopped and the mixtureallowed to set up in the system.

SUMMARY OF THE INVENTION The present invention provides an improvedmethod and arrangement for using a centrifugal impeller to providesuperior mixing of one material into another. An important feature isthat the impeller is positioned inside a mixing container, and themethod includes the steps of directing a portion of the material in thecontainer inward for flow into the center of the impeller and directingat least a portionof the material flowing from the impeller outward athigh velocity for dispersion into the material in the container.

The recirculating flow of material through the impeller and thesecondarymixing that takes place in the container of the present apparatussubstantially improve the rate and degree of mixing over that obtainable with a centrifugal impeller of the same characteristics whenmounted in a conventional pump casing.

In a preferred embodiment of the apparatus of the invention a means fordirecting a portion of the material in the container inward for flowinto the center of the impeller includes a first member preferably inthe form of a circular plate, positioned on one side of the impeller andextending from an outer circumference at least as great as the outercircumference of the impeller to an inner opening coaxial with theaxisof rotation of the impeller. The plate prevents the material in thecontainer from entering the impeller except by flowing inward andthrough the inner opening to the center of the impeller.

For most applications of the invention, the material in the containerwill be a liquid. Additional material, usually in the form of dryingredients, is added by feeding it through a conduit having an outletcoaxial with and adjacent to the pump inlet. Preferably, the conduitoutlet terminates in a flange, spaced axially from the circular plate toform a second member that defines with the first member opposed surfacesfor directing a portion of the first material from the container inwardto mix with the added ingredients. as they leave the conduit.

An important advantage of this arrangement is that the material from thecontainer, as it flows into the central opening of the first member inresponse to the pumping action of the rotating impeller, creates amoving liquid funnel that draws the added ingredients into and throughthe impeller with minimum opportunity for caking and thus choking theimpeller inlet.

For still greater mixing effectiveness, an alternate embodiment of theinvention provides momentum augmentation by recirculating a portion ofthe mixture flowing from the impeller directly back for mixing with thematerial from the container and the added ingredients as they enter intothe center of the impeller.

Still another embodiment, particularly suited for mixing gaseousmaterial into a liquid, includes a venturi section coaxially surroundingthe feed conduit to pro vide pressure augmentation at the impellerinlet, thus avoiding cavitation as a result of adding the gaseousmaterial. This augmented PICSSLIlI'C is obtained by directing a portionof the mixture'flowing from the centrifugal impeller to the inlet of theventuri section. Material from the container also flows inward to mixwith the recirculated mixture entering, the venturi inlet and thematerial added through the conduit.

Accordingly, it is an object of the invention to provide method andapparatus for producing improved mixing action by means of a centrifugalimpeller.

It is still another object of the invention to provide a method andapparatus for mixing dry material into a liquid which produces a uniformmixture in the shortest possible time.

Still another object of the invention is to provide a centrifugalimpeller apparatus for mixing gases into liquid materials withoutproducing cavitation at the impeller inletr Further objects andadvantages of the present invention will become apparent from aconsideration of the drawings in connection with the followingdescription of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevation view of apreferred embodiment of the present invention.

FIG. 2 is a plan view along section lines 2-2 of FIG. 1.

FIGS. 2A and 2B are alternate embodiments of FIG. 2.

FIG. 3 is an elevation view of an alternate embodiment of the invention.

FIG. 4 is an elevation view in section of another alternate embodimentof the invention.

FIG. 5 is an'elevation view in section of still another alternativeembodiment of the invention.

FIG. 6 is an elevation view in section of still another alternativeembodiment of the invention.

FIG. 7 is a section of an alternate blade tip impeller casing design forcreating a toroidal vortex in the mixture leaving the impeller.

FIG. 8 is an elevation view in half-section of interchangeable feedconduit and cover plate embodiments.

FIG. 9 is an elevation view in section of an alternate embodiment formixing multiple ingredients.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2, thepresent invention includes a container 10 having a centrifugal impeller11 mounted for rotation on a shaft 12 extending through the bottom ofthe container and driven by an electrical motor 13 through pulleys 14and 15 by V-belt l6. Shaft l2.is journaled for rotation in bearing 17attached to the bottom of the container. A liquid seal 18 preventsleakage from the container between shaft 12 and bearing 17. A conduit 19for feeding a second material 20 into the center of the impeller extendscoaxially with the axis of impeller rotation. At the upper end ofconduit 19 is a funnel-shaped inlet 21. Conduit 19 terminates at anoutlet 22 adjacent to the inlet 23 of impeller 11.

A supply pipe 24 permits filling container 10 with a first fluidmaterial 25 to a desired level prior to feeding the second materialthrough conduit 19.

A structure such as circular wall 26 surrounds the circumference ofcentrifugal impeller 11, the inner surface 27 of wall 26 being radiallyspaced from the impeller to provide rotational clearance. Passageways 28are circumferentially spaced around wall 26 with inlets 29 adjacent theimpeller and outlets 30 opening into the container.

A first member, such as circular plate 31, is mounted on top of wall 26.The outer circumference of plate 31 is at least as great as the outercircumference of the impeller, and the plate has a central opening 32coaxial with the impeller to provide an inlet to the center of theimpeller. Inner surface 33 of plate 31 is axially spaced from faces 34of impeller blades 35 to provide sufficient clearance for rotation ofthe impeller.

A second member such as flange 36 is connected in coaxially spacedrelation to plate 31 by guide vanes 37 and, in turn, supports the outletof conduit 19 by means of threaded coupling 38. Guide vanes 37 serve toproperly direct the flow of material from the container into theimpeller and also to prevent vortex circulation of the bulk of thematerial in the container that would otherwise reduce mixingeffectiveness.

First member 31 and second member 36 define between them a passagewayfor directing fluid material in the chamber inwardly and downwardlythrough opening 32 into the center of impeller 11.

In operation, container 10 is first filled to a desired level with afluid material from supply pipe 24, and the electric motor is started tocause circulation of the first fluid material inward between members 31and 26, through opening 32 into the center of impeller 11, and thenoutward through impeller vanes 35 and passageways 28 back to mix thematerial in the container. A second material 20, usually a dry powderedor granular material, is poured into funnel 21 and thence feeds throughconduit 19 for mixture with inflowing material 25 at the outlet of theconduit. The two materials then flow together through opening 32,outward between impeller blades 35, and then out through passageways 28into the container. After the desired amount of second material 20 hasbeen added, the mixer can be run for as long as desired, and then theresulting mixture can be drained off through outlet 39.

Alternatively, the apparatus can be operated as a continuous rather thanas a batch mixer by maintaining continual flow through supply pipe 24,conduit 19, and outlet 29 at rates depending upon the ratio of materialsand degree of mixing desired for the finished product.

The advantages of the preferred embodiment in providing improved mixingby means of recirculation through the impeller and secondary mixing inthe container are clearly apparent from FIGS. 1 and 2. First of all, thedesign of the present invention permits intimate mixing of the first andsecond materials before they enter the center of the impeller. As aconsequence, both materials travel together through the impeller blades.The material flowing from the impeller is then directed in a pluralityof high velocity streams through passageways 28 to mix intimately withand to set up secondary circulating currents in the remaining fluid incontainer 10.

As mentioned previously, an additional advantage accruing from theinward flow of material 25 through opening 32 is that the inflowingmaterial 25 creates a moving liquid funnel that sucks material 20 intothe impeller as it is added, without permitting it to cake or stick toinlet 32.

It should be noted that circular wall 26, in conjunction withpassageways 28, insures that there will be less momentum loss of themixture flowing from impeller 11 than occurs in the typical scroll-typepump casing having a tangential outlet; so that the mixture flowsthrough passageways 28 with a relatively high velocity, and theoutflowing jets disperse fully and completely in the surrounding fluidin the container.

The passageways 28 have been depicted in FIG. 2 as being straight andradial, but alternatively they may be angled to impart a swirling,stirring motion to the fluid in the container. They may be angled all inthe same direction or alternating in opposite directions, depending uponthe type of secondary mixing action desired. Referring to FIGS. 2a and2b, passageways 28 of FIG. 2 may be replaced by passageways 28a in FIG.2a or nozzles 28b as in FIG. 2b. Passageways 28a in FIG. 2a are createdby removal of the inner surface of wall 26a in the form of an increasingspiral leading to the inlet of each passageway, the diameter of thespiral increasing in the direction of impeller rotation. Thisarrangement permits a portion of the kinetic energy of the mixtureflowing out from the impeller to be converted to pressure in theincreasing spiral region and then to be reconverted to high velocitykinetic energy in passing through outlets a. The nozzles of FIG. 2, onthe other hand, are useful in situations where it is desired to havevery high velocity mixing jets directed into the material in container10. This arrangement is particularly useful when mixtures are allowed tosettle and then must be stirred up again.

Referring to FIG. 3, an alternate embodiment of the apparatus of FIG. 1provides momentum augmentation for improved mixing of thicker typeslurries and the like. Similar components are designated by the samenumbers as in the previous figures. The embodiment of FIG. 3 includesmeans such as passageways for recirculating a portion of the mixtureflowing from the impeller directly back for mixing with and impartingmomentum to the firstand second materials as they flow into the centerof the impeller. Passageways 40 are formed by at least two conduitmembers 41 having circumferentially spaced inlets 42 adjacent thecircumference of the impeller and circumferentially spaced outlets 43adjacent outlet 22 of conduit 19 for feeding the first material into thecenter of the impeller. Outlets 43 are preferably in the form ofconverging nozzles 44 for producing higher velocity jets and, therefore,greater momentum augmentation. lnlets 42 and outlets 43 are preferablyequally spaced about the impeller circumference and conduit outlet 22,respectively, to provide balanced flow. Outlets 43 may be directedeither in radial planes to intersect the axis of rotation of impeller11, or they may be angled tangentially to produce a swirling motioneither in the same or opposite direction to the rotation of theimpeller, as desired for optimum mixing of specific materials.

Referring next to FIG. 4, still another embodiment, particularly usefulfor mixing gases into liquids, combines a venturi type jet pump with thecentrifugal impeller mixing apparatus of the previous embodiments. InFIG. 4, a venturi section 45 coaxially surrounds central conduit 46adapted for supplying gas for mixing with a liquid material in container10. The outlet 47 of venturi section 45 is located adjacent to theoutlets 48 of conduit 46 and connects with the central opening in plate31 leading to the center of impeller 11. Means such as tubes 49 directat least a portion of the mixture flowing from the impeller through aconverging nozzle 50 to enter inlet 51 of the venturi section as a highve locity stream. As this stream enters venturi 45, it draws a portionof the liquid in the container inward for mixing with a gas from theoutlet of conduit 46 as the liquid enters the centerof the impeller. Theoutlet of conduit 46 should be preferably far enough downstream in thediverging portion of the venturi so that the ambient pressure will tendto force the gas bubbles into solution.

The combination of a jet pump with the centrifugal impeller in theembodiment of FIG. 4 is particularly advantageous when mixing gases intoa liquid because the added pressure provided by the jet pump at theinlet to the impeller tends to prevent cavitation, thus maintaining bothpumping and mixing efficiency through the impeller. v

In all the above embodiments, the second material fed through thecentral conduit and the portion of the first material directed inwardfor mixingwith the sec- 0nd material as it enters the center of theimpeller are supplied from the same side of the impeller. FIG. 5illustrates another embodiment of the invention in which the secondmaterial is fed through conduit 19 into the top of impeller 52, and thefirst material from container 10 is directed inward between the bottomof container 10 and cover plate 53, spaced from the container bottom bysupport 54, for flow upward into the bottom of the impeller. Additionalportions of the first material flow inward through holes in conduit 19in this embodiment, but these holes can be eliminated, if desired. As inthe previous embodiments, the two materials mix as they enter the centerof the impeller and flow outward together through the passagewaysdefined by the impeller blades. Impeller 52 in FIG. 5 is shown with aclosed face, but it can also be an open face type, which has theadvantage of being easier to clean.

Referring to FIG. 6, still another embodiment of the invention uses thecounter flow principle of FIG. 5 in self-contained mixing apparatus thatrequires no connection to or rotary seals through container 10. For thisreason, the apparatus of FIG. 6 is particularly adaptable to portablemixing applications such as hand-held domestic mixers or tank-mountedindustrial mixers. In the embodiment of FIG. 6, conduit 55 is rigidlyconnected to impeller 56, and both are mounted for rotation byelectrical motor 57 through V-belt 58 and pulleys 59 and 60. Conduit 55is supported for rotation by bearings 61 and 62 in tubular supportmember 63. Support member 63 terminates at its lower end in housing 64having passageways 65 leading from adjacent the circumference ofimpeller 56 to the material in the container. Bottom cover plate 66having center opening 67 provides a safety cover to prevent injury byrotating impeller 56, and legs 68 permit circulation of fluid fromcontainer 10 into the center of impeller 56 for mixing with a secondmaterial added through conduit 55. As in the embodiment of FIG. 5,material from the tank may be supplied to the impeller optionallythrough holes 71 in tubular support member 63 and holes 72 throughrotating feed conduit 55 in addition to the upward flow through opening67.

Turning next to FIG. 7, the outer ends 73 of impeller blades 35 and theinner surface 74 of circular wall 26 may optionally be curved ratherthan straight as is shown in the previous embodiments. Curved ends 73and curved surface 74 combine to create a toroidal space ofsubstantially circular cross section that encourages the formation of atoroidal vortex in the material leaving the impeller blades, asindicated by the arrows. The shearing forces set up in the mixture bythis vortex improve the mixing effectiveness of the impeller for atleast some materials.

FIG. 8 illustrates the flexibility of the apparatus of the presentinvention in accommodating mixtures with a wide range of viscosities ina mixer having an impeller of a given size. The right half of FIG. 8 isa half-section of an embodiment 75 adapted for mixing materials ofrelatively low viscosity, and the left side of the figure is ahalf-section of an alternate embodiment 76 for mixing materials ofrelatively high viscosity. For each embodiment, the half section notshown is symmetrical with the half-section shown.

Embodiment 75 includes a feed conduit 77 connected to a circular plate78 by machine screw 79 passing through combination guide vane and spacer80.

Circular plate 78, in turn, is mounted on annular wall member 81 bymachine screws 82. Wall member 81 is similar to wall 26 in FIG. 1, andall other elements of the mixer in FIG. 8 correspond to similar elementsin the previously described mixers.

In embodiment 75 the outlet diameter of conduit 77 and the diameter ofinlet hole 83 through the center of plate 78 are substantially equal tothe diameter of the eye of impeller 84. These represent approximatelythe minimum practical diameters for these openings.

Embodiment 76 includes a feed conduit 84 connected to a circular plate85 by machine screw 86 passing through combination guide vane and spacer87 as in the arrangement of embodiment 75. Plate 85 is attached to wallmember 81 by the same machine screws 82 as are used for plate 78 inembodiment 75. Thus, the two embodiments can be easily interchangedmerely by removing screws 82 and replacing one embodiment with theother.

In embodiment 76, the outlet diameter of feed conduit 84 and thediameter of central inlet hole 88 through plate 85 are a relativelylarge percentage of the outside diameter of the blades of impeller 84.For example, the difference in diameters may be as little as one inch,so that the mixture of material added through conduit 84 and thematerial entering inlet hole 88 from the container flows primarily onlythrough the outer one-half inch of the impeller blades. As mentionedabove, embodiment 76 will handle much more viscous mixtures withoutclogging than will embodiment 75, both because the diameters of conduit84 and inlethole 88 are larger than the corresponding diameters ofconduit 77 and inlet hole 83 and because the vertical spacing betweenthe outlet of conduit 84 and plate 85 is greater than the correspondingspacing between conduit 77 and plate 78.

A family of feed conduits, vane-spacers, and cover plates spanning arange of sizes between embodiments 75 and 76 can provide combinations tomeet a variety of mixing requirements. Different height spacers usedwith a given size feed conduit and cover plate inlet, for example, willcontrol the ratio of recirculation flow to added ingredient rate. Byproviding equal spacing and thread size for the screw holes in all thecomponents, interchangeability is assured.

FIG. 9 illustrates still another arrangement of the mixing apparatus ofthe present invention which combines coaxial feed conduits for twoliquid or solid ingredients plus a gas. A first ingredient, which may bea solid dye, for example, is added through a funnel opening 89 in anouter coaxial feed conduit 90. Simultaneously, a second ingredient(e.g., a liquid dye) is added through a funnel opening 91 of anintermediate coaxial feed conduit 92. Optionally, or in addition, agaseous ingredient can be added through inner coaxial feed conduit 93.The three feed conduits are supported in spaced relation by guide vanes94 although equivalent supporting means can be used. The gaseousingredient can also be added through a central passageway 95 in theimpeller shaft instead of or in addition to supplying it through innerfeed conduit 93.

An important feature of the embodiment of FIG. 9 is that the gaseousingredient is added within the eye of the impeller as close as possibleto the base plate, rather than at the inlet to the cover plate as inFIG. 4. In this way full advantage is taken of the maximum headavailable from the recirculated material in the container so thatpressure augmentation by the venturi device of FIG. 4is not required toprevent cavitation.

The mixing effectiveness of the method and apparatus of the inventionhas been demonstrated by a prototype device similar to that shown inFIG. I and having the characteristics listed in Table I.

TABLE 1 Container Motor Impeller Housing Feed Tube Guide Vanes EXAMPLE Asaturated sugar syrup (62-67 percent sugar by weight) was made by thefollowing procedure:

a. pour 2 /2 quarts of cold water (F into the container,

b. turn motor on and adjust speed for half-power (6 amps),

c. pour 10 pounds of granulated sugar into feed tube as fast as possiblewithout overflowing (approximately 30 seconds between starting to pourand completion with no material remaining in the tube),

(1. increase motor speed to about -90 percent power (9 amps) and run for4% minutes.

As soon as all the sugar is poured through the feed tube, it instantlymixes and uniformly disperses through the water. At this point it is notdissolved, however, and will settle out if the mixture is allowed tostand. After 4 /2 minutes additional mixing (i.e., total 5 minuteselapsed time) the sugar is completely dissolved, and the mixture hasincreased in temperature to approximately 80 F. No sugar willsubsequently recrystallize and settle out of solution, regardless of thetime the mixture is allowed to stand.

From the foregoing descriptions of the various embodiments of thepresent invention, it is clear that all provide a unique and improvedmixing action. An important feature of the present invention is thatmixing takes place within the storage container, thus, eliminating pipe,fittings and valves. By placing the centrifugal impeller within thecontainer, improved mixing action is obtained both through recirculationof a portion of the fluid in the container through the impeller asmaterial is added and throughsecondary mixing action obtained from highvelocity streams of the mixed ingredients leaving the impeller. Previouscentrifugal mixers have used conventional centrifugal pumps to providethe mixing action. For maximum pumping efficiency, these pumps aredesigned with scroll type cases to convert the high momentum of thefluid leaving the impeller into high pressure of the fluid leaving thepump casing. This conversion produces inefficient mixing. On the otherhand, the apparatus and method of the prescut invention maintain thehigh kinetic energy of the fluid streams leaving the impeller and usethis kinetic energy to obtain improved secondary mixing of the fluid inthe container.

I claim:

1. An apparatus for mixing a second material'into a first fluid materialof the type including a motor-driven rotating centrifugal impeller and aconduit having an outlet coaxial with and adjacent to the centrifugalimpeller for feeding the second material into the center of the impellerfor outward flow therethrough, the improvement comprising:

a container for the first material, the impeller being positionedwithinthe container for rotation about an axis coaxial with the outletof the conduit for feeding the second material into the impeller;

a solid structure completely enclosing the impeller for separating theimpeller from the rest of the interior of the container except for atleast one inlet opening through the structure concentric with theimpeller axis of rotation for admitting first fluid material from thecontainer and second material from the outlet of the feed conduit to thecenter of the impeller, the structure having an outer surfacesurrounding the inlet opening from the container that is adapted fordirecting a portion of the first fluid from the container inward formixing with the second material as the second material enters the centerof the impeller, and at least two outlet passageways through thestructure for directing at least a portion of the mixture flowing fromthe periphery of the impeller in a corresponding number of high velocitystreams for dispersion in the fluid in the container, the spacingbetween adjacent outlet passageways being substantially greater than thesize of the passageways so that each high velocity stream leaves therespective outlet passagewayas a separate discrete jet substantiallyspaced from adjacent streams.

2. The mixing apparatus of claim 1 wherein said outer surfacesurrounding the inlet opening from the container is located axiallybetween the impeller and the outlet of the conduit for feedingthe-second material, and the apparatus further comprises:

a second structure inside the container spaced axially from said outersurface of the first mentioned structure, said second structure beingattached to the outlet of the conduit for feeding a second material intothe center of the impeller and having a surface surrounding the conduitoutlet in spaced facing relation to said outer surface of the firstmentioned structure, whereby the first and second structures defineopposed surfaces for directing a portion of the first material from thecontainer inward to mix with the second material as it leaves the outletof the conduit, the mixture then flowing through the inlet opening inthe first structure and into the center of the impeller.

3. The mixing apparatus of claim 1 wherein the structure completelyenclosing the centrifugal impeller comprises:

a circular wall portion in radially spaced surrounding relation to thecircumference of the impeller, the wall having a portion of the innersurface removed I in the form of an increasing spiral leading to theinlet of each passageway in the direction of impeller rotation.

4. The mixing apparatus of claim 1 further comprising:

a plurality of guide vanes mounted parallel to the axis of impellerrotation and extending outward from adjacent the coaxial outlet of theconduit for the second material for controlling the direction of flow ofthe portion of the first fluid from the container into the center of theimpeller and to control vortexing.

5. The mixing apparatus of claim 1 wherein the structure completelyenclosing the centrifugal impeller comprises:

a circular wall in radially spaced relation to the circumference of theimpeller, the wall having a concave inner surface in vertical crosssection, the outer ends of the impeller blades being oppositely curvedso as to define a toroidal space of approximately circular cross sectionbetween the ends of said blades and the inner surface of said wall.

6. A method of mixing a second material into a first fluid material in acontainer comprising the steps of:

directing a portion of the first material to form an inward flowingstream into the center of a rotating centrifugal impeller in thecontainer;

feeding the second material into the center of the impeller so that itcombines with the stream of first material as the first material entersthe center of the impeller; and

directing the mixture of first and second materials flowing from theimpeller to form a plurality of discrete, angularly spaced, highvelocity streams mixing with and dispersing in the fluid in thecontainer, the spacing between adjacent streams being substantiallygreater than their cross-sectional dimensions to produce jet-likestirring and mixing action with the fluid in the container.

7. The method of claim 6 further comprising the step recirculating aportion of the mixture flowing from the impeller to form a plurality ofhigh velocity streams flowing inward toward the center of the impellerto augment the momentum of said portion of thefirst material flowinginto the center of the impeller.

8. An apparatus for mixing a second material into a first fluid materialcomprising:

a container for the first material;

a centrifugal impeller mounted near the bottom of the container forrotation about an approximately vertical axis, the impeller having aplurality of angularly spaced blades extending outwards from an innercircumference forming an open eye to an outer circumference;

drive means for rotating the impeller about the approximately verticalaxis;

a conduit having an outlet in the container coaxial with and spacedabove the upper faces of the cen trifugal impeller blades for feedingthe second material into the open eye of the impeller;

a structure positioned in the container substantially horizontally andspaced between the outlet of the conduit and the upper face of theimpeller, the structure extending outward at least as far as the outercircumference of the impeller and downward to enclose the periphery ofthe impeller, the structure having a central inlet opening coaxial withthe axis of rotation and a plurality of spaced outlet passagewaysleading from a region surrounding the periphery of the impeller back tothe container, the spacing between adjacent outlet passageways beingsubstantially greater than the cross-sectional dimension of thepassageways, fluid material in the container being directed by the uppersurface of the structure to flow inwardly toward the central inletopening for mixing with the second material as the second materialleaves the outlet of the conduit, the mixture then being drawn into theeye of the impeller and forced outward through the impeller and at leasta portion of the mixture flowing from the impeller through the spacedoutlets to emerge in a plurality of high velocity streams for dispersionin the fluid in the container.

9. The apparatus of claim 8 wherein the structure positioned in thecontainer between the outlet of the conduit and the impeller comprises acircular flat plate.

10. The apparatus of claim 9 wherein the conduit for feeding the secondmaterial into the center of the impeller comprises a vertical tubecoaxial with the axis of rotation of the impeller.

11. The apparatus of claim 9 wherein the structure further comprises anannular member surrounding the periphery of the impeller and joining thecircular flat plate positioned between the outlet of the conduit and theimpeller, the annular member having a plurality of holes with inletsequally spaced around the inner circumference of the member and outletsopening into the container for directing high velocity streams flowingfrom the impeller through the holes for dispersion into and secondarymixing with the fluid in the container.

12. A method of mixing a second material into a first fluid material ina container, the method comprising the steps of:

rotating a centrifugal impeller having a number of blades extendingoutwardly in spaced relation from a hollow central eye about an axisconcentric with the eye of the impeller, the impeller being positionedin the container below the surface of the fluid; directing a portion ofthe fluid in the container in a stream that is substantially symmetricalabout the axis of rotation, the stream having a radial component inwardtoward the axis of rotation and an axial component into the eye of theimpeller; introducing the second material into the flowing stream as itenters the eye of the impeller for mixing with the fluid before it isturned outward by the rotating impeller; and forming at least a portionof the mixture of first and second materials flowing from the impellerinto a plurality of spaced high velocity streams for dispersion in thefluid in the container, the spacing between adjacent streams beingsubstantially greater than their cross-sectional dimensions to'produce ajet mixing action between the outflowing streams and the fluid in thecontainer.

1. An apparatus for mixing a second material into a first fluid materialof the type including a motor-driven rotating centrifugal impeller and aconduit having an outlet coaxial with and adjacent to the centrifugalimpeller for feeding the second material into the center of the impellerfor outward flow therethrough, the improvement comprising: a containerfor the first material, the impeller being positioned within thecontainer for rotation about an axis coaxial with the outlet of theconduit for feeding the second material into the impeller; a solidstructure completely enclosing the impeller for separating the impellerfrom the rest of the interior of the container except for at least oneinlet opening through the structure concentric with the impeller axis ofrotation for admitting first fluid material from the container andsecond material from the outlet of the feed conduit to the center of theimpeller, the structure having an outer surface surrounding the inletopening from the container that is adapted for directing a portion ofthe first fluid from the container inward for mixing with the secondmaterial as the second material enters the center of the impeller, andat least two outlet passageways through the structure for directing atleast a portion of the mixture flowing from the periphery of theimpeller in a corresponding number of high velocity streams fordispersion in the fluid in the container, the spacing between adjacentoutlet passageways being substantially greater than the size of thepassageways so that each high velocity stream leaves the respectiveoutlet passageway as a separate discrete jet substantially spaced fromadjacent streams.
 2. The mixing apparatus of claim 1 wherein said outersurface surrounding the inlet opening from the container is locatedaxially between the impeller and the outlet of the conduit for feedingthe second material, and the apparatus further comprises: a secondstructure inside the container spaced axially from said outer surface ofthe first mentioned structure, said second structure being attached tothe outlet of the conduit for feeding a second material into the centerof the impeller and having a surface surrounding the conduit outlet inspaced facing relation to said outer surface of the first mentionedstructure, whereby the first and second structures define opposedsurfaces for directing a portion of the first material from thecontainer inward to mix with the second material as it leaves the outletof the conduit, the mixture then flowing through the inlet opening inthe first structure and into the center of the impeller.
 3. The mixingapparatus of claim 1 wherein the structure completely enclosing thecentrifugal impeller comprises: a circular wall portion in radiallyspaced surrounding relation to the circumference of the impeller, thewall having a portion of the inner surface removed in the form of anincreasing spiral leading to the inlet of each passageway in thedirection of impeller rotation.
 4. The mixing apparatus oF claim 1further comprising: a plurality of guide vanes mounted parallel to theaxis of impeller rotation and extending outward from adjacent thecoaxial outlet of the conduit for the second material for controllingthe direction of flow of the portion of the first fluid from thecontainer into the center of the impeller and to control vortexing. 5.The mixing apparatus of claim 1 wherein the structure completelyenclosing the centrifugal impeller comprises: a circular wall inradially spaced relation to the circumference of the impeller, the wallhaving a concave inner surface in vertical cross section, the outer endsof the impeller blades being oppositely curved so as to define atoroidal space of approximately circular cross section between the endsof said blades and the inner surface of said wall.
 6. A method of mixinga second material into a first fluid material in a container comprisingthe steps of: directing a portion of the first material to form aninward flowing stream into the center of a rotating centrifugal impellerin the container; feeding the second material into the center of theimpeller so that it combines with the stream of first material as thefirst material enters the center of the impeller; and directing themixture of first and second materials flowing from the impeller to forma plurality of discrete, angularly spaced, high velocity streams mixingwith and dispersing in the fluid in the container, the spacing betweenadjacent streams being substantially greater than their cross-sectionaldimensions to produce jet-like stirring and mixing action with the fluidin the container.
 7. The method of claim 6 further comprising the stepof: recirculating a portion of the mixture flowing from the impeller toform a plurality of high velocity streams flowing inward toward thecenter of the impeller to augment the momentum of said portion of thefirst material flowing into the center of the impeller.
 8. An apparatusfor mixing a second material into a first fluid material comprising: acontainer for the first material; a centrifugal impeller mounted nearthe bottom of the container for rotation about an approximately verticalaxis, the impeller having a plurality of angularly spaced bladesextending outwards from an inner circumference forming an open eye to anouter circumference; drive means for rotating the impeller about theapproximately vertical axis; a conduit having an outlet in the containercoaxial with and spaced above the upper faces of the centrifugalimpeller blades for feeding the second material into the open eye of theimpeller; a structure positioned in the container substantiallyhorizontally and spaced between the outlet of the conduit and the upperface of the impeller, the structure extending outward at least as far asthe outer circumference of the impeller and downward to enclose theperiphery of the impeller, the structure having a central inlet openingcoaxial with the axis of rotation and a plurality of spaced outletpassageways leading from a region surrounding the periphery of theimpeller back to the container, the spacing between adjacent outletpassageways being substantially greater than the cross-sectionaldimension of the passageways, fluid material in the container beingdirected by the upper surface of the structure to flow inwardly towardthe central inlet opening for mixing with the second material as thesecond material leaves the outlet of the conduit, the mixture then beingdrawn into the eye of the impeller and forced outward through theimpeller and at least a portion of the mixture flowing from the impellerthrough the spaced outlets to emerge in a plurality of high velocitystreams for dispersion in the fluid in the container.
 9. The apparatusof claim 8 wherein the structure positioned in the container between theoutlet of the conduit and the impeller comprises a circular flat plate.10. The apparatus of claim 9 wherein thE conduit for feeding the secondmaterial into the center of the impeller comprises a vertical tubecoaxial with the axis of rotation of the impeller.
 11. The apparatus ofclaim 9 wherein the structure further comprises an annular membersurrounding the periphery of the impeller and joining the circular flatplate positioned between the outlet of the conduit and the impeller, theannular member having a plurality of holes with inlets equally spacedaround the inner circumference of the member and outlets opening intothe container for directing high velocity streams flowing from theimpeller through the holes for dispersion into and secondary mixing withthe fluid in the container.
 12. A method of mixing a second materialinto a first fluid material in a container, the method comprising thesteps of: rotating a centrifugal impeller having a number of bladesextending outwardly in spaced relation from a hollow central eye aboutan axis concentric with the eye of the impeller, the impeller beingpositioned in the container below the surface of the fluid; directing aportion of the fluid in the container in a stream that is substantiallysymmetrical about the axis of rotation, the stream having a radialcomponent inward toward the axis of rotation and an axial component intothe eye of the impeller; introducing the second material into theflowing stream as it enters the eye of the impeller for mixing with thefluid before it is turned outward by the rotating impeller; and formingat least a portion of the mixture of first and second materials flowingfrom the impeller into a plurality of spaced high velocity streams fordispersion in the fluid in the container, the spacing between adjacentstreams being substantially greater than their cross-sectionaldimensions to produce a jet mixing action between the outflowing streamsand the fluid in the container.